I. Field of the Invention
This invention relates to a drive circuit for a remote transmitter in a process control loop. More specifically, the invention relates to a drive circuit for driving signals across a cable from a microcontroller to a sensing apparatus located at a remote site.
II. Discussion of the Prior Art
Measurement transmitters sensing process variable information, such as differential pressure, line pressure, and temperature, are known. The transmitters are typically used in a process control industry installation where the transmitter power requirement is a concern. Measurement transmitters provide a current output representative of the variable they are sensing, where the magnitude of the current varies between 4-20 mA as a function of the sensed process variable. The current needed to operate a measurement transmitter must be less than 4 mA in order for the transmitter to adhere to this process control industry communications standard.
Most prior art transmitters are required to be located in the immediate vicinity of the location where the process variable information is being sensed. This is due primarily to the low power constraints imposed on the transmitter by the 4-20mA standard, in which the operating current of the measurement transmitter must be less than 4 mA. One way to place the sensing electronics at a longer distance from the measurement transmitter is to use a transmission cable between the sensing electronics and the measurement transmitter. The transmission cable acts as a capacitive load, requiring substantially more current to drive signals between the measurement transmitter and sensing electronics, while maintaining adequate rise and fall times. The longer the transmission cable is, the greater the capacitive load becomes. As a result, prior art transmitters have been required to be placed very close to the process variable being sensed. Often, the sensor electronics are, at most, a few inches from the transmitter components.
To meet certain Intrinsic Safety standards, the sensing electronics must be "infallibly" electrically isolated from the rest of the transmitter. Such standards are set forth by, for example, European CENELEC standards EN50014 and 50020, Factory Mutual Standard FM3610, the Canadian Standard Association, the British Approval Service for Electrical Equipment in Flammable Atmospheres, the Japanese Industrial Standard, and the Standards Association of Australia. The Intrinsic Safety requirements are intended to guarantee that instrument operation or failure cannot cause ignition if the instrument is properly in an environment that contains explosive gases. This is accomplished by limiting the maximum energy stored in the transmitter in a worst case failure situation. Excessive energy discharge may lead to sparking or excessive heat which could ignite an explosive environment in which the transmitter may be operating.
The prior art has primarily used two techniques to achieve infallible isolation between the sensor circuitry and the transmitter circuitry. The first technique is to provide sufficient mechanical segregation or spacing in the sensor such that it is impossible for a component failure to cause electrical shorting to another component or ground. the second technique is to design the entire system such that isolation is not required by using components which are rated for large power dissipation such that they themselves are considered infallible.
One problem with both of these techniques is that they require a sufficiently large transmitter housing to provide the required spacing between components or the relatively large size of the high power components. Thus, reduction in transmitter size has been limited when complying with Intrinsic Safety requirements using the above two techniques.
III. Objects and Advantages
Accordingly, it is a principal object of the present invention to provide an improved system for interfacing a measurement transmitter or microcontroller with a peripheral, such as a sensing electronics capsule, at a remote sensing site or location while still adhering to the 4-20 mA industry standard.
A further object of the invention is to provide an intrinsically safe cable drive circuit so that operation can be maintained even in hazardous gas environments.